With dwindling petroleum resources, increasing energy prices, and environmental concerns, development of energy efficient biorefinery processes to produce biobased chemicals and materials from renewable, low cost, carbon resources offers a unique solution to overcoming the increasing limitations of petroleum-based chemicals.
Fuels, plastics, and chemicals derived from agricultural feedstocks are receiving considerable attention as the world looks for solutions to dwindling non-renewable petroleum resources (Herrera, Nature Biotechnol. 24:755-760 (2006); Kamm et al., Adv. Biochem. Eng. Biotechnol. 105:175-204 (2007); Ragauskas et al., Science 311:484-489 (2006)). In the United States, efforts have primarily focused on biofuels such as ethanol produced from the starch in maize kernels. Efforts in other parts of the world like Brazil use sugar feedstocks from sugarcane. Currently there are extensive efforts to develop new sources of feedstocks for ethanol production including cellulose hydrolysate from biomass.
Polyhydroxyalkanoates (PHAs), a family of naturally renewable and biodegradable plastics, occur in nature as a storage reserve in some microbes faced with nutrient limitation (Madison et al., Microbiol. Mol. Biol. Rev. 63:21-53 (1999)) and possess properties enabling their use in a variety of applications currently served by petroleum-based polymers. PHAs can be extracted from microorganisms or genetically engineered crops and used in polymer form as PHA biobased plastics or can be chemically or thermally converted to a range of renewable chemicals. PHA biobased plastics can be produced via commercial large scale fermentations of microbial strains and the marketing of these plastics in a variety of applications is well under way (Bieles, Plastics and Rubber Weekly, February 17:1 (2006)). Since they are inherently biodegradable in soil, compost, and marine environments, they can decrease plastic waste disposal issues.
Existing fermentation methods for producing polyhydroxyalkanoates utilize wild-type or transgenic microorganisms cultured on specific substrates to produce the desired PHA polymer composition. In many cases the polymers of interest are copolymers of the (D)-isomer of 3-hydroxybutyrate copolymerized with one other 3, 4 or 5-hydroxyacids. These copolymers are produced as granular inclusions inside the cells and are random copolymers. Existing fermentation methods generally use clean sugars, fatty acids or vegetable oils as the primary feedstock for the microorganisms. Secondary feedstocks are usually supplied to enable the incorporation of the second monomer.
The availability and cost of corn or cane sugars as a feedstock can be prohibitively high especially when growing microbes on an industrial scale. Therefore there is a need to find lower cost carbon substrates for producing PHAs via microbial fermentation processes.